1. Technical Field of the Invention
This invention relates to a voltage-change control circuit and electronic apparatus (e.g. camera) including such a voltage-change control circuit.
2. Description of the Related Art
Conventionally, there is known a charge-pump booster circuit used in generating a write/erase voltage to/from a flash memory, for example, or on a high-voltage generating circuit for a liquid-crystal display on a video camera, digital camera or the like.
The charge-pump booster circuit has a series connection of a plurality of stages of pumping packets, each configured by one capacitor and diode, to generate a voltage higher than, e.g., a power voltage VDD of an LSI chip by boosting the voltage in each pumping packet (note that boosting in this case means a boosting in absolute value level, i.e. positively boosting for a positive polarity of power voltage, and negatively boosting for a negative polarity of power voltage).
In addition, the charge-pump booster circuit encounters a noise problem (pumping noise) which occurs when driving a clock buffer for a pumping clock or a pumping capacitor during a pumping operation.
For this reason, the pumping noise of a booster circuit has an effect upon the analog signal of an imaging device, for example. Thus, there is a concern with respect to image noise appearing over an imaging screen or a variation in write distribution under the influence of pumping noise during writing a flash memory or the like.
FIG. 4 is a block diagram showing the general configuration example of a booster circuit. As shown in the figure, the booster circuit has a charge-pump booster circuit section 10 configured by capacitors C1–C4 and diodes D1–D5 and a clock buffer section 20 configured by a plurality of inverter-type buffer amplifiers I1–I5. Note that, in each inverter-type buffer amplifier I1–I5, VDD represents a power-sided voltage while VSS represents a ground-sided voltage.
In the booster circuit thus configured, when a clock signal is inputted, e.g. to the capacitor C1, the voltage at a node ND1 is boosted in an amount of voltage VDD (ideal value) by the capacitance coupling of the capacitor C1.
The voltage (V1) at ND1 at that time, if ideally considered, is:V1=VDD+αwhere α is the initial potential at the node ND1.
Herein, the initial potential a differs, between the nodes ND1–ND4, depending upon the threshold voltage change, the initial state or the like of the diodes D1–D4. However, the boost voltage is common to VDD.
The VDD provides a change amount of boost voltage, which varies at a step VDD before a desired boost voltage is reached.
Incidentally, in the foregoing explanation relates to boosting up the positive polarity of voltage. However, when boosting down a negative polarity of voltage in absolute value level, the voltage at each node ND1–ND4 is boosted down by a similar clock buffer.
In the conventional booster circuit, however, when boosting up or down for the capacitor of booster circuit by the use of a clock having a VDD voltage width, a potential change of VDD step (VDD−VSS) takes place even at a point close to the desired potential. Even if controlled by a regulator, the potential will vary based on the VDD step.
Meanwhile, driving the capacitor and clock buffer section on the VDD voltage causes useless power, having an effect as a great noise source.
Furthermore, boost voltage cannot be controlled without using a potential change of VDD width=VDD step.
Accordingly, it is desired to control voltage on a small step basis to thereby suppress boost-voltage variation, and further to reduce the noise upon boosting up or down for the capacitor as well as the noise due to the clock buffer.
Therefore, it is an object of the present invention to provide a voltage-change control circuit and method capable of controlling, on a small-step basis, the control clock on the booster circuit, thereby suppressing the occurrence of various noises.